Propellers and Choosing Propellers
- Smile Khatri
Choosing a right propeller is necessary to ensure that sufficient thrust is generated to propel the aircraft forward during flight. This document goes over how propellers work, how they are dimensioned, and considerations for choosing a propeller.
Propeller Action
Propellers are essentially vertically mounted rotating wings (RC Airplane Propeller Size Guide (rc-airplane-world.com)). The thrust created by propellers is analogous to the lift created by wings of the plane (except since the propeller is mounted vertically, it is referred to as thrust). The faster the RPM, the greater the thrust generated, just as how lift is proportional to velocity^2 of the airfoil.
However, unlike the wings of an aircraft, propellers have a ‘twist’ from the root (hub) to the tip of the blade. The blade of a propeller is made up of a series of airfoils that are oriented at differing angles of attack along the length of the blade (see the image below). The air speed near the root of the propeller is different than at the tip of the blade. The velocity of a point along a circular motion is given by the formula below:
where Vr = tangential velocity, r = radius, and ω = rad/s (ω = RPM*2*pi / 60)
Therefore, the further a point on a propeller blade is from the root, the faster the air speed at that point. Since the lift coefficient is a function of angle of attack (and therefore lift force as well), the airfoils along the propeller blade must be twisted to allow for various angle of attacks along the blade to generate constant thrust. For example, the airfoil at the tip of the blade would have the smallest angle of attack but since it would experience the fastest wind speed it still generates sufficient thrust.
As propellers rotate, the velocity of air accelerates and a slipstream is created and air swirls around the fuselage. When this slipstream strikes other parts of the plane, like the vertical stabilizer, extra drag can be induced.
Propeller Dimensions
Propellers are sized in their diameter (tip of one blade to tip of another blade) and pitch (as shown below). Pitch indicates how far that propeller moves through the air per a single revolution. However, pitch is only a guide, the actual advance of propeller is affected by the prop conditions, air density, and other factors. Higher pitch means more advance. Propellers may sometimes be called ‘air screws’ because they cut through air and move forwards.
In general:
for high speed flight, a combination of smaller diameter + higher pitch is suitable
for slower flight, a combination of larger diameter + lower pitch is suitable
Keep in mind that the larger the prop diameter, the slower the RPM because the motor must work more. However, since smaller propellers rotate faster, they generate more noise as the tip of the blade cuts through the air.
Choosing a Propeller
There are various types of propellers available, 2 blade, 3 blade, …. The more the number of blades, the less efficient the propeller because each blade has to cut through turbulent air from the preceding blade.
It is always the most efficient to use the largest propeller diameter that the motor can safety accommodate (Aircraft Engine and Propeller Sizing | AeroToolbox). This is because a larger diameter prop allows thrust to be generated at slower RPM, whereas a small diameter propeller would need higher RPM.
If the motor manufacturers have specific requirements for props, make sure they are obeyed.
The following video can be a starting point for sizing the propeller:
ION STREAM EN 16x9 (youtube.com)
The presenter provides the following empirical relationships:
prop_diameter (in inches) = -0.002 * RPM + 35.607 (round down if needed)
prop_pitch = prop_diamter / 1.57 (round to the nearest half)